1. Field of the Invention
The present invention relates generally to a recording medium allowing high-density recording and reproduction of information, and more particularly to a magneto-optical recording medium and a manufacturing method therefor.
2. Description of the Related Art
A magneto-optical disk is known as a high-density recording medium, and an increase in its recording density is demanded with an increase in quantity of information. Increasing the recording density of the medium can be realized by shortening the space between adjacent recording marks. However, the reproduction of each recording mark is limited by the size of a light beam (beam spot) on the medium. In the case that the density of the recording marks is set so that only one recording mark is present inside the beam spot, an output waveform corresponding to xe2x80x9c1xe2x80x9d or xe2x80x9c0xe2x80x9d can be observed as a reproduced signal according to whether or not the recording mark is present inside the beam spot.
However, in the case that the density of the recording marks is increased so that a plurality of recording marks are present inside the beam spot, the reproduced output does not change irrespective of movement of the beam spot on the medium, so that the output waveform becomes linear and the presence or absence of recording marks cannot be distinguished. The reproduction of such small recording marks having a period shorter than the size of the beam spot may be effected by reducing the size of the beam spot. However, the size of the beam spot is limited by the wavelength xcex of light output from a light source and the numerical aperture NA of an objective lens, so that the spot size cannot be sufficiently reduced.
Recently commercially available is a magneto-optical disk drive adopting a reproducing method using a magnetically induced super-resolution (MSR) technique for reproducing a recording mark smaller than the size of the beam spot by the use of an existing optical system. The MSR is a reproducing method such that while one mark present inside the beam spot is being reproduced, another mark is masked to thereby increase a reproductive resolution. Accordingly, such an MSR medium requires at least a mask layer or reproducing layer for masking the other mark so that the one mark is reproduced during signal reproduction, in addition to a recording layer for recording marks.
A magneto-optical recording medium using a perpendicularly magnetized film as the reproducing layer is proposed in Japanese Patent Laid-open No.3-88156, for example. In the prior art described in this publication, however, an initial magnetic field of several kilooersteds is required for initialization of the reproducing layer. Accordingly, the disk drive using this magneto-optical recording medium cannot be reduced in size. A magneto-optical recording medium using a magnetic film having an axis of easy magnetization in a longitudinal direction at room temperature and an axis of easy magnetization in a perpendicular direction at a predetermined temperature or higher as the reproducing layer is proposed in Japanese Patent Laid-open No. 5-81717, for example.
Further, an MSR medium having a reproducing layer, a recording layer, and an intermediate layer interposed between the reproducing layer and the recording layer is described in U.S. Pat. No. 6,020,079. In the MSR medium described in this U.S. patent, a reproducing laser beam is directed onto the medium to form a temperature distribution composed of a low-temperature region, an intermediate-temperature region, and a high-temperature region inside the beam spot. The low-temperature region and the high-temperature region form a double mask, and a recorded mark is read from only the intermediate-temperature region. Since the double mask is formed by the low-temperature region and the high-temperature region, the intermediate-temperature region for reading a recorded mark can be greatly reduced in size, thereby allowing high-density recording and reproduction. Such a magneto-optical recording medium is referred to as a double-mask rear aperture detection (DRAD) type magneto-optical recording medium.
Also known is an MSR medium having a nonmagnetic intermediate layer of SiN or the like between the reproducing layer and the recording layer. In this MSR medium, a temperature distribution composed of a low-temperature region and a high-temperature region is formed in a beam spot by directing a reproducing laser beam onto the medium, thereby forming a mask in the low-temperature region to read a recorded mark from the high-temperature region. This magneto-optical recording medium is referred to as a center aperture detection (CAD) type magneto-optical recording medium.
The DRAD type or CAD type MSR medium as mentioned above has an advantage such that the recording density is higher than that of the conventional magneto-optical medium, so that the storage capacity can be increased. However, the tolerance of the composition of the reproducing layer or the recording layer is narrow. In particular, it is a remarkable problem that the tolerance of the composition of the reproducing layer is narrow. The recording layer and the reproducing layer in a magneto-optical recording medium are generally formed by sputtering, and composition variations of about xc2x10.5 at % generally occur in normal sputter deposition. However, the tolerance of the composition in an MSR medium is narrower than xc2x10.5 at %, so that the characteristics of the medium become unstable and it is difficult to produce the MSR medium with its quality being maintained.
In general, each layer of a magneto-optical recording medium such as an MSR medium is formed by setting an alloy target in a deposition chamber of a stationary opposed type sputtering device and performing sputter deposition. The above-mentioned composition variations of about xc2x10.5 at % are considered to be due to a difference between individual alloy targets and an aged deterioration by the use of an alloy target.
As a method for solving this problem, it is known that a rare earth target of Gd or the like and a transition metal target of FeCo or the like are prepared and these two targets are set in the same deposition chamber to simultaneously perform sputtering. According to such a two-element simultaneous sputtering method, the composition of each layer can be adjusted by controlling sputter powers to be applied to the two targets, thereby allowing the correction of the composition variations due to the individual difference or aged deterioration of targets.
However, this two-element simultaneous sputtering method has problems that a sputtering device for carrying out this method becomes large in size and the availability is reduced, causing an increase in manufacturing cost. In contrast, the stationary opposed type sputtering device using alloy targets has an advantage such that the size of the device is small and the availability is high. However, in the conventional manufacturing method using the stationary opposed type sputtering device, the composition variations cannot be corrected.
It is therefore an object of the present invention to provide a low-cost and high-quality recording medium and a manufacturing method therefor.
It is another object of the present invention to provide a magneto-optical recording medium suitable for high-density recording which can reduce a reproducing magnetic field and a crosstalk and improve a C/N.
It is a further object of the present invention to provide a magneto-optical recording medium manufacturing method which can manufacture a high-quality magneto-optical recording medium at a low cost.
In accordance with an aspect of the present invention, there is provided a magneto-optical recording medium comprising a magnetic recording layer for recording information; and a magnetic reproducing layer provided on said magnetic recording layer for reading information; said magnetic reproducing layer comprising at least one first reproducing layer having a first composition and at least one second reproducing layer having a second composition slightly different from said first composition, said first and second reproducing layers having the same principal ingredients.
Preferably, said magnetic reproducing layer contains Gd, the difference in Gd composition between said first reproducing layer and said second reproducing layer being in the range of 0.5 to 3.0 at %. More preferably, the difference in said Gd composition is in the range of 0.7 to 2.0 at %. Preferably, said magnetic reproducing layer is composed of GdFeCo, and said Gd composition is in the range of 24.0 to 27.0 at %. Preferably, the total thickness of said first reproducing layer and said second reproducing layer is in the range of 35 to 60 nm. More preferably, this total thickness is in the range of 40 to 50 nm.
Preferably, the magneto-optical recording medium is a DRAD type magneto-optical recording medium having a magnetic intermediate layer between the magnetic recording layer and the magnetic reproducing layer. Alternatively, the magneto-optical recording medium is a CAD type magneto-optical recording medium having a nonmagnetic intermediate layer between the magnetic recording layer and the magnetic reproducing layer.
In accordance with another aspect of the present invention, there is provided a manufacturing method for a magneto-optical recording medium, comprising the steps of forming a first magnetic reproducing layer having a first composition by sputtering; forming a second magnetic reproducing layer having a second composition slightly different from said first composition on said first magnetic reproducing layer by sputtering, said first and second magnetic reproducing layers having the same principal ingredients; and forming a magnetic recording layer on said second magnetic reproducing layer by sputtering; variations in composition of said first and second magnetic reproducing layers being corrected by changing a ratio in film thickness between said first and second reproducing layers.
Preferably, the first and second magnetic reproducing layers and the magnetic recording layer are formed by using a stationary opposed type sputtering device. Preferably, each of said first and second magnetic reproducing layers is composed of GdFeCo, the composition of Gd contained in each of said first and second magnetic reproducing layers being in the range of 24.0 to 27.0 at %. More preferably, the difference in said Gd composition between said first magnetic reproducing layer and said second magnetic reproducing layer is in the range of 0.5 to 3.0 at %.
In accordance with a further aspect of the present invention, there is provided an information recording medium comprising at least one first recording layer having a first composition; and at least one second recording layer having the same principal ingredients as those of said first recording layer and a second composition slightly different from said first composition.
The present invention is applicable to a general information recording medium such as an optical disk and a magnetic disk. By changing a ratio in film thickness between said first and second reproducing layers, variations in composition of said first and second magnetic reproducing layers can be corrected.
The above and other objects, features and advantages of the present invention and the manner of realizing them will become more apparent, and the invention itself will best be understood from a study of the following description and appended claims with reference to the attached drawings showing some preferred embodiments of the invention.